Changing from one grade of polymer to another requires a transition period for a polymerization reactor to switch over to new resin specifications and corresponding process conditions such as reaction temperature, reactants and reactant ratios. During the transition from one product to another, off-grade polymer material is produced that does not have the desired resin flow property (e.g., melt index), density, or other property of either the initial product or the desired target product. In addition, a polymerization reaction operating under “steady state” conditions can encounter variations that can result in the production of off-grade polymer material that can lead to loss of revenue and reactor shutdown. Since off-grade polymer material presents an economic loss, it is desirable, to minimize the length of time a reactor produces such material and the amount of material that is produced.
A number of methods have been described to reduce transient, off-grade polymer material. Such methods have involved feeding a polymerization retarder or catalyst poison (e.g., CO2, O2) into the reactor, adjusting reaction gas composition, temperature and possibly pressure to new values, removing reactant gases from the reactor, reducing the catalyst feed rate, and/or adding a nonreactive gas such as nitrogen, among other remedial actions.
Despite existing approaches to limit off-grade material, there is a continuing need and desire to provide a more effective and efficient process to reduce the amount of off-grade polymer material produced during the transition to a new product or as a result of a fluctuation during steady state manufacture.